1. Technical Field
The present specification relates to an image forming apparatus, particularly to an image forming apparatus for forming an image on a sheet and including a conveying belt for electrostatically attracting the sheet.
2. Discussion of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, plotters, or multifunction printers having at least one of copying, printing, scanning, plotting, and facsimile functions, typically form an image on a recording medium (e.g., a sheet). Thus, for example, a recording head moving in a main scanning direction discharges ink droplets onto a sheet being conveyed in a sub-scanning direction to form an image on the sheet.
In one example of such image forming apparatus, a conveying belt is looped over at least two rollers, for example, an upstream roller and a downstream roller, and conveys a sheet. A charging roller is driven by the conveying belt and charges the conveying belt. A pressing member opposes the upstream roller provided upstream in a sheet conveyance direction to press the sheet on the conveying belt against the conveying belt. A recording head discharges ink droplets onto the sheet electrostatically attracted to and conveyed by the conveying belt to form an image on the sheet. A sheet discharge device is provided downstream from the conveying belt in the sheet conveyance direction to discharge the sheet bearing the image to an outside of the image forming apparatus. For duplex printing, a duplex unit reverses and re-feeds the sheet formed with an image on a first side (e.g., a front side) thereof by the recording head so that an image can then be formed on a second side (e.g., a back side) of the sheet.
The conveying belt conveys the sheet intermittently. Thus, it is preferable to drive intermittently also the sheet discharge device, which is provided downstream from the conveying belt in the sheet conveyance direction, in synchronization with the intermittent conveyance of the sheet by the conveying belt.
However, a problem can arise when the sheet discharge device intermittently conveys the sheet at a speed identical to or slower than a speed at which the conveying belt intermittently conveys the sheet, in that slack may arise in the sheet fed from the conveying belt and result in a sheet jam. Alternatively, the sheet may separate from the conveying belt and consequently a faulty image may be formed on the sheet.
To address this possible problem, the sheet discharge device intermittently conveys the sheet at a speed faster than the speed at which the conveying belt intermittently conveys the sheet. However, in so doing, the sheet discharge device may pull the sheet with a force greater than an electrostatic attraction force of the conveying belt when the sheet passes through a nip portion formed between the conveying belt and the pressing member, at which point the sheet is held in place on the conveying belt solely by the electrostatic attraction force of the conveying belt. Consequently, the sheet discharge device and the conveying belt may not intermittently convey the sheet with respect to the recording head properly, resulting in formation of a faulty image or a low-quality image.
Further, in duplex printing, a sheet bearing an image on the front side thereof may warp when liquid droplets forming the image are dried. As a result, the sheet may not be properly attracted to the conveying belt, resulting in formation of a faulty image or a low-quality image on the back side of the sheet.